The present invention relates to a process for coating and laminating film using 100% solids adhesive at room temperature.
Traditionally, when preparing some packaging intended to be used to package food products, film is printed on one side, adhesive is applied on top of the printing as well as on the clear areas surrounding the printing, and then the film is laminated to a second film at a laminating nip. In in-line printing, the adhesive is coated on top of the freshly printed ink or the opposite web and immediately thereafter is laminated.
On web machines having a width of 30 inches to 60 inches or more, coating is typically accomplished by employing rotogravure and flexo coating, although other techniques may be used. In many cases, lamination is accomplished xe2x80x9cout of linexe2x80x9d, meaning that a printed roll is taken to another location for lamination.
In the early years of lamination of such packaging materials, solvent borne polyurethane adhesives were employed that gave good xe2x80x9cgreenxe2x80x9d tack and cured further to create good laminations. Over the last 20 years, water borne adhesives have replaced many of the solvent-based adhesives due to the reduction of emissions of noxious vapors (VOCs). Of course, solvent-based adhesives are still employed. Water borne adhesives include urethanes, acrylics and other hybrids.
As the industry has evolved, the desire to reduce solvent emissions created a demand for water-based adhesives. However, a 100% solids polyurethane chemistry using a very expensive four or five roll coating head evolved and is currently making big inroads into the market for laminating adhesives. Such systems are prevalent in Europe and are gaining more acceptance in the United States. When the four or five roll coating head systems are used, no oven is employed for drying but most of the adhesives, if not all of them, must be kept hot or heated during application because the adhesives have high viscosities. As should be understood, heating the adhesives reduces the viscosity. However, at the same time, application of heat severely reduces pot life. In other words, application of heat initially reduces viscosity but speeds the initiation of the exothermic reaction that results in adhesive curing and rapid viscosity increase. The pot life of an adhesive is the time period from the moment it is mixed until the moment the exothermic curing reaction begins to make it unusable. When reaction begins, viscosity rapidly increases.
The adhesives employed with the four or five roll coating system are typically two component systems that must be mixed. Such adhesives are adversely affected by moisture in the air and their higher viscosity, in the range of 1000 cps or more, requires use of the four or five roll coating system.
In the normal use of 100% solids urethane adhesives, there is a major health issue that has surfaced in which the urethanes can pose a health hazard when used in laminating film for flexible packaging used for foodstuffs. In essence, the isocyanate in the adhesive reacts and can form aromatic diamine which is a suspected health hazard and which can also migrate through the film.
For some convertors using the four or five roll system, they heat the polyurethane adhesives as high as 150xc2x0 to 160xc2x0 F. in order to obtain a runnable viscosity. When such heating is done and the film is run at a speed of over 500 feet per minute, misting can occur which fouls many of the parts of the machinery, and contaminates the atmosphere around the machine thereby creating a potential health hazard for the operator of the machine. To prevent this problem, often, the machinery must be slowed down to prevent misting. Although, four and five roll coating systems can run adhesives having a viscosity of 1,000 cps or higher, the misting issue must be addressed in order to effectively coat film while avoiding expensive machinery upkeep and danger to the health of the machine operator.
Often, products are sold using a xe2x80x9cjust in timexe2x80x9d inventory storage procedure and, due to this procedure, demand is increasing for packaging that may be printed and laminated on narrow web presses such as those having a width of 26 inches or less. One example of such an application is the creation of bottle labels of film-film laminations. Flexographic presses are often used to print images on the film. In such systems, in order to make the presses capable of laminating, a system was devised to use an ultraviolet cured adhesive having a low enough viscosity to allow coating by flexographic presses with curing being accomplished immediately thereafter using an ultraviolet light source. Ultraviolet curing has a severe limitationxe2x80x94it cannot be used to cure adhesive on a metallized web combined with a reverse printed web because the ultraviolet light will not penetrate the ink or metal. The same is true where the film is white or opaque.
As is well known in the art, water or solvent-based adhesives are not preferred to be used on flexographic presses because it is quite difficult to apply a sufficient amount of adhesive in a flexographic press and such adhesives are difficult to dry. With typical printing, it has been found that the only way to use water or solvent-based adhesives in a flexographic press is to slow the speed of production or equip with a much larger drying oven. Limitations on production speed are problematic. Thus, the ultraviolet curing system is a preferred system for use with narrow width presses with the UV cured adhesive having a viscosity of about 300 cps.
Such a system works adequately for ultraviolet bottle label applications but has been found to be unacceptable for applications where one of the films comes into contact with a food product. It has been found that the ultraviolet photo-initiators do not fully respond to the ultraviolet light and, as such, the U.S. Food and Drug Administration has not accepted such adhesives for use in indirect food packaging.
Manufacturers of solvent borne adhesives have attempted to make very high solids (65%) adhesives that can be coated on a gravure press. Such an application reduces the emissions but has not been commercially successful. Water-based adhesives comprising up to 60% solids have been developed with such configuration helping in drying and coating weight but only limited applications have been found for such adhesives.
The adhesive disclosed in Parent application Ser. No. 09/699,437 includes, in some formulations, a flexible epoxy known as DOW 736. Applicant has found that this flexible epoxy causes emission of odors that preclude use of the inventive adhesive in applications where the foodstuff has the ability to absorb odors. One example of such a foodstuff is coffee. As such, a need has developed for an adhesive that will effectively adhere films together without odor emission.
Applicants are aware of U.S. Pat. No. 4,216,252 to Moeller which discloses a solventless release coating. While the solventless nature of the release coating is analogous to the adhesive employed in the inventive process of the present invention, the particular coating employed is significantly different from the adhesive employed herein, and is used for a different purpose.
Each of the techniques and adhesives described above presents significant limitations of one kind or another, whether it be noxious emissions, including misting, requirement for quick production, difficulties in uniform curing, heating requirements coupled with short pot lives, etc. Additionally, some adhesives can cause smudging or bleeding of some inks used to print on the films to be laminated which can cause poor print quality on the finished packaging. As such, a need has developed for a coating technique that employs an adhesive having a sufficiently low enough viscosity at room temperature to permit even and predictable flow, wherein the adhesive does not emit noxious fumes, is not affected by moisture, where pot life is extended to a sufficient degree to increase efficiency of production, and wherein the adhesive employed in the process is deemed acceptable by the U.S. Food and Drug Administration for use in laminating packaging to be used to package food products, and wherein smudging or bleeding of the inks may be reduced.
It is with these problems and needs in mind that the present invention was developed.
The present invention relates to a process for coating and laminating film using 100% solids adhesive at room temperature. The present invention includes the following interrelated objects, aspects and features:
(1) In a first aspect, Applicants have found that a superior system for conducting the inventive process consists of a two doctor blade coating system. In such a system, two doctor blades combine with other structure to form an enclosed chamber which receives adhesive. The adhesive is pumped into the chamber and, within the chamber, engages a portion of the circumference of a roll having a surface texture. While this roll may be described as xe2x80x9cknurledxe2x80x9d, it is more accurately called an anilox roll or gravure cylinder. It may be engraved or ceramic coated with one of several patterns laser etched into the surface. This roll will be described hereinafter as xe2x80x9cknurledxe2x80x9d and is considered to contemplate all of the alternatives described above. The chamber has an exit allowing adhesive to be recirculated during conducting of the process. A circulating pump is employed that pumps adhesive from a sump to the enclosed chamber and the sump receives adhesive recirculated back from the enclosed chamber. Adhesive is mixed and supplied to the sump while the process is being conducted so that the enclosed chamber is continuously full of adhesive.
(2) In the system employed in conducting the inventive process, neither the blades nor the nip are heated and the process is conducted at room temperature. The blades may be water cooled to offset heat generated by frictional interaction between the blades and knurled roll to maintain adhesive temperature at desired temperature.
(3) In one example of the operation of the inventive process, in order to make sure that the chamber defined by the doctor blades is always maintained full of adhesive, the recirculating pump is designed to pump adhesive at a flow rate twice the rate of usage of adhesive by the system.
(4) One example of an adhesive that Applicants have found suitable for use in conducting the inventive process is a two-stage epoxy having part A and part B with the mixing ratio being two parts by weight of part A mixed with one part by weight of part B, although this ratio may be varied.
(5) The adhesive preferably includes the following constituent ingredients:
Part A: 51% by weight Aliphatic DIEPOXIDE Epoxy Resin, 34% by weight Bisphenol A Type Epoxy Resin, 1% Nonionic defoamer (Example: Air Products DF-75), 13% Dipropylene Glycol Dibenzoate, and 1% by weight Silicone Free surfactant (Example: Air Products SEF).
Part B consists of 79% by weight Amidoamine curing agent, 20% by weight Aliphatic Amidoamine curing agent, 0.5% by weight defoamer as described above, and 0.5% by weight surfactant as described above.
The proportions of the constituent ingredients may be varied as will be described in greater detail hereinafter.
(6) The adhesive is a zero VOC 100% solids laminating adhesive having a mixed viscosity of preferably 200-1500 cps with the viscosity remaining within a desired range at room temperature, defined as a temperature range of 50-110 degrees F for up to 3 hours. In certain applications, such as where a four or five coating method is employed, the viscosity can be adjusted to be as high as 1500 cps. The adhesive is solventless and is unaffected by moisture. Applicants have found that curing of the adhesive will take place for most substrates within 48 hours. The adhesive may be cleaned using IPA (isopropyl alcohol) for at least six hours after mixing and can be cleaned using toluene up to 1 day after mixing. As an important aspect, the adhesive consists of ingredients which make it permissible for use as an adhesive for laminations of films designed for food packaging pursuant to F.D.A. Regulation 175.105. The inventive adhesive does not emit strong odors.
(7) Any adhesive that possesses the properties of the adhesive as described above and which complies with F.D.A. Regulation 175.105 would be a suitable substitute therefor in the inventive process.
(8) Hereinabove, in the BACKGROUND OF THE INVENTION, Applicants discussed the difficulty of trying to coat high solids adhesive using gravure coating. Such high solids adhesive typically comprises urethane. An advantage of the present invention is the ability to coat 100% solids adhesive by gravure coating technique. Such a technique may be performed both in-line, usually by applying the adhesive to an unprinted web with its own oven, as well as out-of-line on a separate machine. Such a technique can also be accomplished in the last deck of a flexopress as would be done on a narrow machine. The wide web application is extremely important since the wider web increases the amount of surface area per unit time that flows through the machine. Thus, the wider web consumes a much higher amount of adhesive than narrow web applications. Through the use of the present invention, by eliminating the adhesive drying oven, one may double production from, for example, 400 to 500 feet per minute of film to 800 to 1000 feet per minute of film. Additionally, the use of the inventive adhesive in the inventive process reduces costs since energy costs expended for drying the adhesive in an oven are eliminated.
(9) In a typical in-line wide web printing and laminating operation, the coating method is gravure. Such a method of application facilitates putting a heavier coating weight down to obtain satisfactory bonding, but requires a large oven to dry the solvent or water from the adhesive. Again, through the teachings of the present invention, the oven is eliminated. The etch of the knurled cylinder determines the coating weight. Through the use of the present invention, the etch is much finer than usual since only solids are applied instead of solids plus carrier, whether the carrier is a solvent or water. Of course, additionally, the hot nip is also eliminated, which also eliminates curling of the finished product due to the avoidance of exposure to heat both in the oven and in the nip.
(10) In such a system, the print ovens are very small with short dwell time as compared to the size and dwell time of the adhesive oven. This disparity is due to the thinness of the ink coating as compared to the thickness of the adhesive coating. In narrow web flexo-operations where water-based adhesive is applied, high solids adhesives are used in order to obtain a coating weight that is sufficiently heavy enough to work while being easy to dry. In the gravure process, one fewer roll is used as compared to the flexoprocess. In gravure, two rolls are used whereas in flexo, three rolls are used.
(11) The process in accordance with the teachings of the present invention can also be used in extrusion coating. In extrusion coating, the adhesive can be applied by gravure and when the hot extrudate hits the adhesive during the laminating process, it is cured in the range of 80% cure instantaneously. As before, the extrusion process does not require the oven. A typical gravure application will apply the adhesive and the web is then conveyed to the nip where hot molten polyethylene, polypropylene or other materials are extruded to the adhesive. If desired, a second web is brought in to laminate with the first-mentioned web, but this is not absolutely necessary. One or both webs can be printed. Such a process is a superior application since high speed process can be undertaken without a drying problem and with instantaneous bonding.
(12) Applicant has found that there is a high interest in 100% solids adhesive technology for use by wide web convertors, namely, four or five roll coating systems. Such systems typically are able to use higher viscosity adhesive products because the plurality of rolls facilitates spreading of the adhesive to a thickness that may be effectively coated despite the relatively high viscosity.
(13) The adhesives disclosed herein may advantageously be used in a four or five roll coating system, even at a high rate of speed, from 500 feet per minute to as much as 1,500 feet per minute, because the inventive adhesives do not create aromatic diamine or other odors and are highly resistant to misting even at such high coating speeds.
(14) The adhesives disclosed herein are highly advantageous over adhesives currently used in coating processes, particularly since they are able to run at room temperature. As should be understood, running at room temperature avoids the expense of providing heaters and supplying them with energy. Furthermore, when one runs at room temperature, the viscosity of the adhesive remains substantially uniform throughout the coating process, thereby maintaining the coating weight substantially uniform. Where the operator feels the need to heat a drum of adhesive, typically, the heating is done externally and creates a temperature gradient in which the adhesive around the periphery of the drum is hotter than the adhesive at the center within the drum. Such non-uniformity creates variance in coating weights that is problematic. The present invention eliminates this problem.
(15) In practicing the teachings of the present invention, Applicant has been required to take viscosity measurements of the inventive adhesives. For this purpose, Applicant has employed a Brookfield Viscometer Model DV2+ using an S-27 spindle at 25xc2x0 C.xc2x11xc2x0.
Accordingly, it is a first object of the present invention to provide a process for coating and laminating film using 100% solid adhesives at room temperature
It is a further object of the present invention to provide such a process wherein the adhesive employed does not emit VOCs (volatile organic compounds).
It is a further object of the present invention to provide such a process wherein the adhesive employed is solventless, possesses low viscosity, and maintains a relatively steady viscosity at room temperature.
It is a still further object of the present invention to provide such a process wherein the adhesive employed is unaffected by moisture and may be used to laminate films to be used as packaging for food products pursuant to F.D.A. Regulation 175.105.
It is a yet further object of the present invention to provide such a process wherein a two doctor blade laminating system is employed wherein adhesive is recirculated to and from a chamber partially defined by the doctor blades.
It is a still further object of the present invention to provide such a process wherein the adhesive employed is resistant to misting and may be formulated to have a viscosity up to 1,500 cps at room temperature.
It is a still further object of the present invention to provide a process in which the inventive adhesive may be used in a four or five roll coating process while avoiding misting and creation of vapors while running at high speeds as much as 500 to 1,500 feet per minute.
These and other objects, aspects and features of the present invention will be better understood from the following detailed description of the preferred embodiment when read in conjunction with the appended drawing figures.